I would say the main reason is that there just aren't as many out there. What Civil War reenactors want heavily influences the manufacturers, and most reenactors want the 1861 because it is more correct for use throughout the war, including the early years. In addition to looks, the 1863 has a simplified and more reliable fire channel in the breech. Actually, the Colt Special model 1861 was patterned after the British Enfield. Colt had previously had a contract to produce Enfield style rifle muskets, so when they got the contract to make model 1861 Springfields they bent the rules a bit and produced them with some of the Enfield tooling. They were accepted, perhaps grudgingly as the muskets were desperately needed.